Acoustic Echo Suppression

ABSTRACT

A controller for an echo suppressor configured to suppress a residual echo of a far-end signal included in a primary error signal, the controller adapted for operation with a primary adaptive filter configured to form a primary echo estimate of the far-end signal included in a microphone signal and an echo canceller configured to cancel that primary echo estimate from the microphone signal so as to form the primary error signal, the controller comprising: a secondary adaptive filter configured to form a secondary echo estimate of the far-end signal comprised in the microphone signal; and control logic operable in at least two modes selected in dependence on a convergence state of the primary adaptive filter, the control logic being configured to control activation of the echo suppressor in dependence one or more transient or steady state decision parameters.

BACKGROUND OF THE INVENTION

This invention relates to a controller for an acoustic echo suppressorand a method of controlling an acoustic echo suppressor.

In telephony, an echo is a reflection of the voice signal. It is adelayed copy of the original. An example scenario is illustrated in FIG.1, which shows a signal being captured by a far-end microphone andoutput by a near-end loudspeaker. The echo is a consequence of acousticcoupling between the loudspeaker and the near-end microphone; thenear-end microphone captures the signal originating from its ownloudspeaker in addition to the voice of the near-end speaker and anybackground noise. The result is an echo at the far-end loudspeaker. Echocancellation is an important feature of telephony. Hands-free devicesand teleconferencing, in particular, require echo cancellation that canadapt to environments having a wide range of acoustic characteristics.

Echo cancellers typically synthesise an estimate of the echo from thefar-end voice signal. The estimated echo is then subtracted from themicrophone signal. This technique requires adaptive signal processing togenerate a signal accurate enough to cancel the echo effectively. Anadaptive filter is often used to model the environment's acousticimpulse response.

An acoustic echo canceller and adaptive filter are described inInternational Patent Application WO 2012/158163, incorporated byreference herein in its entirety. The acoustic echo canceller describedtherein uses a non-linear processor operating in the frequency domain todetermine suppression factors for each of a plurality of frequencybands. The echo canceller uses the suppression factors to control theremoval of echo from a near-end audio signal. However, even though theecho canceller works reasonably well in high echo return loss scenarios,it suffers from poor performance during low echo return loss scenarios(i.e. high ratios of echo to near-end signal). Additionally, thecomputational cost of coherence measures between signals in thefrequency domain is high.

Even with high performance adaptive filters it is not always possiblefor an echo canceller to remove all echoes from a signal, and the echocancelled signal from an echo canceller will often include residual echoof the far-end voice signal. This is because the echo estimate generatedby an adaptive filter will not always precisely match the true echo inthe microphone signal. There can be several reasons for this, includingloss of convergence of the adaptive filter due to changes in echo pathand as a result of freezing the adaptive filter during near-end speechto avoid wide divergence of the filter.

In order to address the problem of residual echo in a microphone signalfollowing echo cancellation, an echo suppressor can be used to removethe residual echo by replacing or masking the microphone signal whenresidual echo is present. To ensure that an echo suppressor is enabledonly at appropriate moments, echo suppressors are typically controlledaccording to the presence of near-end speech. This is with the aim ofavoiding the introduction of artefacts into the microphone signal orotherwise interfering with near-end speech carried in the microphonesignal. U.S. Pat. Nos. 6,507,653 and 6,532,289 describe detectors foridentifying near-end speech and controlling an echo suppressor accordingto whether or not near-end speech is identified. However, usingconventional near-end speech detectors to control an echo suppressor canlead to clipping of double talk during periods of high echo relative tothe near-end speech present in a microphone signal.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided acontroller for an echo suppressor configured to suppress a residual echoof a far-end signal included in a primary error signal, the controlleradapted for operation with a primary adaptive filter configured to forma primary echo estimate of the far-end signal included in a microphonesignal and an echo canceller configured to cancel that primary echoestimate from the microphone signal so as to form the primary errorsignal, the controller comprising:

-   -   a secondary adaptive filter configured to form a secondary echo        estimate of the far-end signal comprised in the microphone        signal, the length of the primary adaptive filter being greater        than the length of the secondary adaptive filter;    -   a coherence estimator configured to form a first measure of        coherence between the microphone signal and the primary error        signal, and a second measure of coherence between the microphone        signal and the primary echo estimate; and    -   control logic operable in at least two modes selected in        dependence on a convergence state of the primary adaptive        filter, the control logic being configured to:        -   in its first mode selected when the primary adaptive filter            is in a non-converged state, combine the microphone signal            and the secondary echo estimate so as to form a transient            decision parameter indicative of a state of the microphone            signal and, in dependence on the transient decision            parameter, control activation of the echo suppressor; and        -   in its second mode selected when the primary adaptive filter            is in a converged state, combine the first and second            measures of coherence so as to form one or more first steady            state decision parameters indicative of a state of the            microphone signal and, in dependence on said one or more            first steady state decision parameters, control activation            of the echo suppressor.

In embodiments of the invention, machine readable code can be providedfor generating the controller. In embodiments of the invention, amachine readable storage medium having encoded thereon non-transitorymachine readable code can be provided for generating the controller.

The echo suppressor may be configured to, when activated, replace theprimary error signal with generated noise having characteristicsselected to substantially match background noise comprised in themicrophone signal.

The transient decision parameter may be indicative of the presence ofecho of the far-end signal in the microphone signal.

The control logic may be configured to, in its first mode, causeactivation of the echo suppressor when the transient decision parameterindicates that echo of the far-end signal is present in the microphonesignal but near-end speech is not present.

The control logic may be configured to, in its first mode, determinethat the transient decision parameter indicates that echo but notnear-end speech is present in the microphone signal when the transientdecision parameter is above a predefined threshold.

The control logic may be configured to, in its first mode, combine themicrophone signal and the secondary echo estimate such that thetransient decision parameter represents a measure of angle between avector expression of the microphone signal and a vector expression ofthe secondary echo estimate, said vector expressions each being asequence of samples of the respective microphone or secondary echoestimate.

(A) The secondary adaptive filter may be configured to form thesecondary echo estimate from the far-end signal and adapted independence on a second error signal generated by means of a comparisonof its secondary echo estimate and the microphone signal.

The secondary adaptive filter may be configured to receive the far-endsignal by means of a delay line arranged to delay the far-end signal bya number of samples commensurate with a measure of the time delaybetween the far-end signal and its echo in the microphone signal.

(B) The secondary adaptive filter may be configured to form thesecondary echo estimate from the primary echo estimate and adapted independence on a secondary error signal generated by means of acomparison of its secondary echo estimate and the microphone signal.

The coherence estimator may be further configured to form a thirdmeasure of coherence of the microphone signal with itself, and thecontrol logic may be configured to, in its first mode, combine themicrophone signal and the secondary echo estimate in dependence on thethird measure of coherence.

The third measure of coherence may be a measure of autocorrelation orenergy of the microphone signal.

The controller may comprise both a first secondary adaptive filtersconfigured in accordance with paragraph (A) and a second secondaryadaptive filters configured in accordance with paragraph (B), thecontrol logic being configured to, in its first mode, form respectivefirst and second transient decision parameters and control activation ofthe echo suppressor in dependence on both the first and second transientdecision parameters.

The one or more first steady state decision parameters may be indicativeof the presence of near-end speech.

The control logic may be configured to, in its second mode, causeactivation of the echo suppressor when the one or more first steadystate decision parameters indicate that near-end speech is not present.

The control logic may be configured to, in its second mode, determinethat the one or more first steady state decision parameters indicatethat near-end speech is not present when the or more first steady statedecision parameters are above a first predetermined threshold.

The first measure of coherence may be a measure of cross correlationbetween the microphone signal and the primary error signal, and thesecond measure of coherence may be a measure of cross correlationbetween the microphone signal and the primary echo estimate.

The control logic may be configured to, in its second mode, combine thefirst and second measures of coherence so as to form a first one of theone or more first steady state decision parameters proportional to adifference between the first and second measures of coherence, saiddifference being scaled by a measure of the magnitude of the microphonesignal.

The control logic may be configured to, in its second mode, combine thefirst and second measures of coherence so as to form a second one of theone or more first steady state decision parameters proportional to adifference between the first and second measures of coherence, saiddifference being scaled by a sum of the first and second measures ofcoherence.

The coherence estimator may be further configured to form a thirdmeasure of coherence of the microphone signal with itself, and thecontrol logic being configured to, in its second mode, combine the firstand third measures of coherence so as to form a second steady statedecision parameter indicative of a state of the microphone signal and tocontrol activation of the echo suppressor further in dependence on thesecond steady state decision parameter.

The control logic may be configured to combine the first and thirdmeasures of coherence such that the second steady state decisionparameter is proportional to a ratio of the first and third measures ofcoherence.

The coherence estimator being further configured to form a fourthmeasure of coherence of the primary error signal with itself, and thecontrol logic being configured to, in its second mode, combine the thirdand fourth measures of coherence so as to form a third steady statedecision parameter indicative of a state of the microphone signal and tocontrol activation of the echo suppressor further in dependence on thethird steady state decision parameter.

The control logic may be configured to combine the third and fourthmeasures of coherence such that the third steady state decisionparameter is proportional to a ratio of the third and fourth measures ofcoherence.

The third measure of coherence may be a measure of autocorrelation orenergy of the microphone signal, and the fourth measure of coherence maybe a measure of autocorrelation or energy of the primary error signal.

The second and third steady state decision parameters are indicative ofthe presence of near-end speech.

The control logic may be configured to, in its second mode, causeactivation of the echo suppressor when the second and/or third steadystate decision parameters indicate that near-end speech is not present.

The control logic may be configured to, in its second mode, determinethat the second and/or third steady state decision parameters indicatethat near-end speech is not present when the second steady statedecision parameter is below a second predetermined threshold and/or thethird steady state decision parameter is above a third predeterminedthreshold.

The controller may further comprise a convergence discriminatorconfigured to identify the convergence state of the primary adaptivefilter in dependence on one or more measures of an expected timerequired for the primary adaptive filter to converge to a predeterminedlevel of convergence.

The convergence discriminator may be further configured to identify theconvergence state of the primary adaptive filter in dependence on one ormore of the steady state decision parameters.

The lengths of the primary and secondary adaptive filters may berepresented by the number of coefficients of the respective adaptivefilter or represented by the length of time corresponding to the numberof samples over which the respective adaptive filter concurrentlyoperates.

The secondary adaptive filter may be configured to operate at a lowersampling rate than the primary adaptive filter.

According to a second aspect of the present invention there is provideda method for controlling an echo suppressor configured to suppress aresidual echo of a far-end signal included in a primary error signalreceived from an echo canceller, the echo canceller being configured tocancel a primary echo estimate from a microphone signal so as to formthe primary error signal, the primary echo estimate being formed at aprimary adaptive filter and representing an estimate of the far-endsignal comprised in the microphone signal, the method comprising:

-   -   at a secondary adaptive filter, forming a secondary echo        estimate of the far-end signal comprised in the microphone        signal, the length of the primary adaptive filter being greater        than the length of the secondary adaptive filter;    -   forming a first measure of coherence between the microphone        signal and the primary error signal, and a second measure of        coherence between the microphone signal and the primary echo        estimate;    -   determining a convergence state of the primary adaptive filter;        and    -   selecting in dependence on the determined convergence state:        -   a transient decision path if the primary adaptive filter is            determined to be in a non-converged state, the transient            decision path combining the microphone signal and the            secondary echo estimate so as to form a transient decision            parameter indicative of a state of the microphone signal;        -   a steady state decision path if the primary adaptive filter            is determined to be in a converged state, the steady state            decision path combining the first and second measures of            coherence so as to form one or more first steady state            decision parameters indicative of a state of the microphone            signal;            and    -   controlling activation of the echo suppressor in dependence on        said transient or one or more first steady state decision        parameters.

In embodiments of the invention, machine readable code can be providedfor implementing the method of switching encode configurations at anencoder pipeline. In embodiments of the invention, a machine readablestorage medium having encoded thereon non-transitory machine readablecode can be provided for implementing the method of switching encodeconfigurations at an encoder pipeline.

The method may comprise, on the transient decision path, combining themicrophone signal and the secondary echo estimate such that thetransient decision parameter represents a measure of angle between avector expression of the microphone signal and a vector expression ofthe secondary echo estimate, said vector expressions each being asequence of samples of the respective microphone or secondary echoestimate.

The step of forming a secondary echo estimate may comprise:

-   -   at the secondary adaptive filter, forming the secondary echo        estimate from the far-end signal; and    -   adapting the secondary adaptive filter in dependence on a second        error signal generated by means of a comparison of its secondary        echo estimate and the microphone signal.

The step of forming a secondary echo estimate may comprise:

-   -   at the secondary adaptive filter, forming the secondary echo        estimate from the primary echo estimate; and    -   adapting the secondary adaptive filter in dependence on a        secondary error signal generated by means of a comparison of its        secondary echo estimate and the microphone signal.

The step of forming a secondary echo estimate may comprise forming afirst secondary echo estimate at a first secondary adaptive filterconfigured in accordance with paragraph (A) and forming a secondsecondary echo estimate at a second secondary adaptive filter configuredin accordance with paragraph (B); and the method may comprise:

-   -   on the transient decision path, forming respective first and        second transient decision parameters; and    -   controlling activation of the echo suppressor in dependence on        both the first and second transient decision parameters.

The first measure of coherence may be a measure of cross correlationbetween the microphone signal and the primary error signal, and thesecond measure of coherence may be a measure of cross correlationbetween the microphone signal and the primary echo estimate.

The method may comprise, on the steady state decision path, combiningthe first and second measures of coherence so as to form a first one ofthe one or more first steady state decision parameters proportional to adifference between the first and second measures of coherence, saiddifference being scaled by a measure of the magnitude of the microphonesignal.

The method may comprise, on the steady state decision path, combiningthe first and second measures of coherence so as to form a second one ofthe one or more first steady state decision parameters proportional to adifference between the first and second measures of coherence, saiddifference being scaled by a sum of the first and second measures ofcoherence.

The method may further comprise:

-   -   forming a third measure of coherence of the microphone signal        with itself;    -   on the steady state decision path, combining the first and third        measures of coherence so as to form a second steady state        decision parameter indicative of a state of the microphone        signal; and    -   controlling activation of the echo suppressor further in        dependence on the second steady state decision parameter.

The step of combining the first and third measures of coherence may beperformed such that the second steady state decision parameter isproportional to a ratio of the first and third measures of coherence.

The method may further comprise:

-   -   forming a fourth measure of coherence of the primary error        signal with itself;    -   on the steady state decision path, combining the third and        fourth measures of coherence so as to form a third steady state        decision parameter indicative of a state of the microphone        signal; and    -   controlling activation of the echo suppressor further in        dependence on the third steady state decision parameter.

The step of combining the third and fourth measures of coherence may beperformed such that the third steady state decision parameter isproportional to a ratio of the third and fourth measures of coherence.

The third measure of coherence may be a measure of autocorrelation orenergy of the microphone signal, and the fourth measure of coherence maybe a measure of autocorrelation or energy of the primary error signal.

The step of determining a convergence state of the primary adaptivefilter may be performed in dependence on one or more measures of anexpected time required for the primary adaptive filter to converge to apredetermined level of convergence.

The step of determining a convergence state of the primary adaptivefilter may be performed further in dependence on one or more of thesteady state decision parameters.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example withreference to the accompanying drawings. In the drawings:

FIG. 1 shows an example of near-end and far-end in telephony.

FIG. 2 is a schematic diagram of a controller for an acoustic echosuppressor.

FIG. 3 is a schematic diagram of decision logic of the controller.

FIG. 4 is a schematic diagram of short filters of the controller.

FIG. 5 is a flowchart illustrating the general operation of thecontroller.

FIG. 6 is a flowchart illustrating an exemplary algorithm performed byconvergence detection logic of the controller.

FIG. 7 is a flowchart illustrating an exemplary algorithm performed bytransient decision logic of the controller.

FIG. 8 is a flowchart illustrating an exemplary algorithm performed bysteady state decision logic of the controller.

FIG. 9 illustrates (a) microphone and error signals, (b) the resultingvariation in transient decision parameters, and (c) the resultingvariation in steady state decision parameters.

FIG. 10 illustrates near-end speech detection delay in a systemcomprising the controller and in a conventional system.

FIG. 11 illustrates double talk detection delay in a system comprisingthe controller and in a conventional system.

FIG. 12 illustrates near-end speech detection delay in a systemcomprising the controller at an ENR of 0 dB.

FIG. 13 illustrates double talk detection delay in a system comprisingthe controller at an ENR of 0 dB.

FIG. 14 illustrates near-end signal attenuation during double talk for asystem comprising the controller and for a conventional system.

DETAILED DESCRIPTION

The following description is presented by way of example to enable anyperson skilled in the art to make and use the invention. The presentinvention is not limited to the embodiments described herein and variousmodifications to the disclosed embodiments will be readily apparent tothose skilled in the art.

There is a need for an improved controller for an acoustic echosuppressor and an improved method of controlling an acoustic echosuppressor.

A controller for an acoustic echo suppressor is provided for operationwith an acoustic echo canceller and primary adaptive filter. Thecontroller may control the echo suppressor in accordance with the outputof a decision path selected in dependence on the convergence state ofthe adaptive filter. The decision paths can be used to determine whenthe suppressor is to be activated in dependence on decision parametersformed for each decision path. The controller may include a secondaryadaptive filter for forming a secondary echo estimate of a far-endsignal in the microphone signal on which the canceller and suppressoroperate. In the examples described herein the secondary adaptive filteris a short adaptive filter.

FIG. 2 shows an example of a controller 202 for an acoustic echosuppressor (AES) 203. The controller and other components shown in FIG.2 could, for example, be provided at a communication device (e.g. a VoIPhandset) operable to communicate with another such communication devicein the manner discussed with respect to FIG. 1. The controller receivesa far-end signal x, a microphone signal d, an estimated echo signal y,and an error signal e representing the difference between the microphonesignal and the estimated echo signal and output by echo canceller 204.The microphone signal is captured by microphone 213 and will typicallycapture echo of the far-end signal x output by loudspeaker 212. The echois a delayed version of far-end signal x filtered by the acousticenvironment, represented by impulse response h.

Also shown in FIG. 2 is an Adaptive Echo Estimation Filter (AEEF) 201configured to estimate the acoustic echo y received at the microphone213 from speaker 212 and provide that estimated echo to the echocanceller 204. The acoustic echo represents leakage from the system'sloudspeaker to its microphone. This is predominantly the acousticimpulse response of the room, but it may also incorporate elementsrelating to the hardware and software in the audio driver interface andother devices in the audio path. The AEEF 201 uses the estimated impulseresponse ĥ to generate the estimate of the echo signal ŷ, which isremoved from the audio signal d captured by the microphone. AEEF 201could be any suitable adaptive filter for forming an estimate of theacoustic echo between speaker and microphone.

Controller 202 can be configured to control the acoustic echo suppressor203 in dependence on the far-end signal x, microphone signal d,estimated echo signal ŷ, and error signal e that it receives. This isachieved by means of two decision paths embodied at the controller andselected in dependence on whether the AEEF 201 is deemed to haveconverged. The decision paths include a transient state decision pathwhich is selected when the AEEF has not converged, and a steady statedecision path which is selected when the AEEF has converged. Each of thedecision paths is adapted for controlling activation of the echosuppressor 203 in dependence on a state of the microphone signal.

For example, the microphone signal may be assigned one of threedifferent states:

-   -   STATE 1: Far-end speech alone and background noise (single talk)    -   STATE 2: Near-end speech with background noise or background        noise alone    -   STATE 3: Both far-end and near-end speech with background noise        (double talk)

In the examples described herein the decision paths of the controllercan be configured to identify periods of residual echo in the microphonethat do not include near-end speech and on which echo suppression shouldbe performed. The controller can be referred to as a non-linearprocessor or NLP since output from the controller is not a linearfunction of its signal inputs and instead derived by means of decisionstaken in accordance with one or more algorithms.

In many situations where the near-end signal contains significant signalenergy that is independent of any echo this will be due to talking atthe near-end. This situation is conveniently denoted “near-end speech”herein. The signal energy might, of course, be due to a different soundsource. This is particularly true during teleconferencing or hands-freeoperation. Therefore, the term “near-end speech” is used to refer to anysignificant signal energy in the near-end signal that is not due to anecho. It should also be understood that the term “near-end speech” islargely synonymous with the term “double-talk”.

Typically the AEEF is constantly adapted only when near-end speech isnot present (e.g. in STATE 1), with adaptation of the AEEF being frozenduring the presence of near-end speech (e.g. in STATE 2 and STATE 3) inorder to avoid divergence of the filter.

The echo estimate ŷ formed by the AEEF 201 is provided to echo canceller204, which subtracts the echo estimate from the microphone signal so asto generate an error signal e. Under steady state conditions when theAEEF is converged and the echo path between speaker and microphone issteady and well-defined, the echo estimate is likely to be accurate andthe error signal will therefore contain very little, if any, echo of thefar-end signal. However, under other conditions, there can be aconsiderable residual echo present in the error signal which has notbeen cancelled by echo canceller 204. This could be because, forexample, the AEEF has not converged or an echo path change has occurred.At appropriate times, an acoustic echo suppressor 203 can be employed tosuppress such residual echoes. For example, the echo suppressor can beconfigured to replace the error signal with synthetic noise generated soas to match the—characteristics of the ambient background noise receivedat the microphone. Such noise can be termed “comfort noise”.Alternatively, the echo suppressor could attenuate the error signalacross all or a subset of frequency bands.

The echo suppressor 203 is activated by controller 202, which can beconfigured to identify regions of residual echo in the error signal thatdo not include near-end voice and which are suitable to be replaced withsynthetic noise. In this manner, a receiver of the error signal (such asa far-end communication device) can be provided with an echo-freeacoustic signal from the microphone. In the embodiments of thecontroller described herein, the echo suppressor is either activated(performing echo suppression) or not activated (not performing echosuppression). However, in other embodiments of the controller, thedegree of activation of the echo suppressor could be controlled by thecontroller—for example, with the AES 203 being controlled to blendsynthetic noise with the error signal so as to mask but not replaceresidual echo.

It can be advantageous to provide the controller with a sample rateconverter (SRC) 205 so as to allow the controller to operate at a lowersampling rate than the audio signals with respect to which thecontroller performs its analysis. This enables the controller to consumefewer resources of the system at which it is implemented. For example,the controller could be configured to operate at a sample rate of 8 kHz,with the microphone signal, far-end signal, error signal and echoestimate having sample rates of 16 kHz or 24 kHz or 32 kHz or 48 kHz(all the audio signals would typically have the same sample rate, butthey could be different). If sample rate down conversion is performed,an interpolator 210 can be provided at the controller so as to upconvertthe control signal from decision logic 209 back to the appropriatesample rate for the error signal on which the AES 203 operates.

The controller 202 controls the activation of the echo suppressor bymeans of decision parameters selected according to whether the AEEF isdeemed to have converged. Thus, a first set of one or more decisionparameters are used in the case that the AEEF has not converged, and asecond set of one or more decision parameters are used in the case thatthe AEEF has converged. Examples of the calculation and use of suchdecision parameters at the controller will now be described.

Firstly, it is useful to consider the echo path impulse response vectorh between speaker and microphone, which can be modelled as:

h=[h ₀ h ₁ h ₂ . . . h _(N-1)]^(T)  (1)

Where N is the length of the echo path, sampled at same samplinginstance as microphone signal d and far-end or reference input signal x.

Let y(n) be the actual acoustic echo, s(n) be the near-end signal andv(n) be the ambient background noise at discrete time instant n. Themicrophone signal can be written as the sum of the actual echo, thenear-end signal and the ambient background noise.

d(n)=y(n)+s(n)+v(n)  (2)

Similar to h, the far-end or reference signal vector x and microphonesignal vector d are given by:

x=[x(n) x(n−1) x(n−2) . . . x(n−(N−1))]^(T)  (3)

d=[d(n) d(n−1) d(n−2) . . . d(n−(L−1))]^(T)  (4)

Where L represents frame length or block length. Its value should not begreater than N.

The actual echo y(n) and the estimated echo 9(n) output by the adaptivefilter are given by the true echo path h and the estimated echo path ĥ.

y(n)=h ^(T) x  (5)

ŷ(n)=ĥ ^(T) x  (6)

The error signal e(n) in the acoustic echo cancellation process is givenby:

e(n)=d(n)−{circumflex over (y)}(n)  (7)

This error signal is often used for adapting the adaptive filter.Commonly this adaptation is achieved using a Normalized Least MeanSquare (NLMS) algorithm with a fixed or adaptive step size p:

$\begin{matrix}{{\hat{h}\left( {n + 1} \right)} = {{\hat{h}(n)} + {2\mu \frac{{e(n)}{x(n)}}{{x}^{2}}}}} & (8)\end{matrix}$

The data processing system may operate on each sample in the time domainor on blocks of samples in the time domain. It may also operate onblocks of samples in the frequency domain or on individual samples in acombination of the time and frequency domains.

In the case of time domain sample-based processing, the acoustic echoestimate ŷ(n) and error signal e(n) for each discrete instant areestimated using corresponding far-end sample x(n) and microphone signald(n). In the case of block-processing, the adaptive filter will operateon each block of the far-end signal. To generalize the implementation tobe either sample-based or block-based, vector representations can beused for the far end data buffer x used by the adaptive filter given by(3), current block of far-end data to be processed x₁ the echo estimateŷ, error e and microphone signal d. These vectors can be given as:

x ₁ =[x(n) x(n−1) x(n−2) . . . x(n−(L−1))]^(T)  (9)

ŷ=[ŷ(n) {circumflex over (y)}(n−1) {circumflex over (y)}(n−2) . . .{circumflex over (y)}(n−(L−1))]^(T)  (10)

d=[d(n) d(n−1) d(n−2) . . . d(n−(L−1))]^(T)  (11)

e=[e(n) e(n−1) e(n−2) . . . e(n−(L−1))]^(T)  (12)

In sample based processing, each sample x(n) in x is fed to a far-enddata buffer for use as an input to the AEEF and the oldest sample isremoved. For block based processing, a chunk of data of length L is fedto the far-end data buffer allocated to x and the oldest samples oflength L are removed.

The error signal from the adaptive filter will often include a residualecho that is not cancelled by the echo estimate generated by the filter.This can be due to the filter not being converged and hence the echoestimate being inaccurate, as well as due to echo path changes thatoccur whilst the filter is frozen (e.g. because near-end speech has beendetected). Such a residual echo is generally unimportant during near-endspeech and double talk (STATE 2 and STATE 3 above) because humanperception is such that a residual echo is not noticeable to a personwho is talking. However, when there is only far end speech andbackground noise in the microphone signal, the residual echo after AEEFcancellation can be significant and dominate over any residualbackground noise in the error signal. This can be particularly true oncethe error signal has been amplified at a far-end receiving devicearranged to receive the processed output from the near-end microphone.The role of the AES 203 is to suppress such residual echoes in the errorsignal.

Estimation of Correlation Parameters

In the present example, four correlation parameters are calculated fromwhich decision parameters can be formed. The correlation parameters areformed at correlation parameter logic 206 of the controller 202.

1. Cross Correlation r_(de) Between the Microphone Signal and the ErrorSignal

The cross correlation r_(de) between microphone signal d and errorsignal e is given by

r _(de)(n)=E[de ^(T)]  (13)

Since the error signal e is the difference between microphone output dand the echo estimate ŷ, r_(de) can be given as.

r _(de)(n)=E[(d)(d−ŷ)^(T)]  (14)

After substituting microphone signal as given in equation (2), r_(de)can be arrived as given below, where s is the near-end signal withoutany background noise or echo, and v is the ambient background noise inthe microphone signal.

r _(de)(n)=E[(y+s+v)(y+s+v−ŷ)^(T)]  (15)

From (4) and (5), substituting actual echo and the echo estimate in theequation (15)

$\begin{matrix}{{r_{dc}(n)} = {E\left\lbrack {\left( {{h^{T}x} + s + v} \right)\left( {{h^{T}x} + s + v - {{\hat{h}}^{T}x}} \right)^{T}} \right.}} & (16) \\{{r_{dc}(n)} = {E\left\lbrack \begin{pmatrix}{{h^{T}{xx}^{T}h} + {sy}^{T} + {{vy}^{T}{ss}^{T}} +} \\{{vv}^{T} - {s{\hat{y}}^{T}} - {vy}^{T} - {h^{T}{xx}^{T}\hat{h}} +} \\{{ys}^{T} + {yv}^{T} + {sv}^{T} + {vs}^{T}}\end{pmatrix} \right\rbrack}} & (17)\end{matrix}$

According to independent theory, near end speech and background noise isassumed to be un-correlated to the echo signal. Hence, their crosscorrelation is assumed to be zero. So, equation (17) can be reduced asgiven below

r _(de)(n)=E[(h ^(T) xx ^(T) h+ss ^(T) +vv ^(T) −h ^(T) xx ^(T) ĥ)  (18)

Taking the auto-correlation of the far-end signal as R_(xx) we can write(18) as:

r _(de)(n)=h ^(T) R _(xx) h+σ _(s) ₂ +σ_(v) ₂ −h ^(T) R _(xx) ĥ  (19)

Where σ_(s) ₂ is the variance of the near-end signal and σ_(v) ₂ is thevariance of the ambient background noise.

2. Cross Correlation r_(dŷ) Between the Microphone Signal and the EchoEstimate

The cross correlation r_(dŷ) between the microphone signal d and theecho estimate is given by

r _(d)(n)=E[dŷ ^(T)]  (20)

Substituting microphone signal and echo estimate from (2) and (5), weget

r _(dŷ)(n)=E[(y+s+v)({circumflex over (y)})^(T)]  (21)

r _(dŷ)(n)=E[(h ^(T) x+s+v)(ĥ ^(T) x)^(T)]  (22)

Based on the independent theory, assuming echo is uncorrelated to thenear end and background noise

r _(dŷ)(n)=E[h ^(T) xx ^(T) ĥ]  (23)

r _(dŷ)(n)=h ^(T) R _(xx) ĥ  (24)

3. Auto-Correlation or Energy of the Microphone Signal

The auto correlation or energy of the microphone signal R_(dd) is givenby

R _(dd)(n)=∥d ² ∥=[dd ^(T)]  (25)

∥d ² ∥=E[(y+s+v)(y+s+v)^(T)]  (26)

∥d ² ∥=E[(h ^(T) xx ^(T) h+ss ^(T) +vv ^(T))]  (27)

R _(dd)(n)=∥d ² ∥=h ^(T) R _(xx) h+σ _(s) ₂ +σ_(v) ₂   (28)

4. Auto-Correlation or Energy of the Error Signal

The auto correlation or energy of the error R_(ee) is given by

$\begin{matrix}{{R_{ee}(n)} = {E\left\lbrack {\left( {d - \hat{y}} \right)\left( {d - \hat{y}} \right)^{T}} \right\rbrack}} & (29) \\{{R_{ee}(n)} = {E\left\lbrack {\left( {y + s + v - \hat{y}} \right)\left( {y + s + v - y} \right)^{T}} \right\rbrack}} & (30) \\{{R_{ee}(n)} = {E\left\lbrack {\left( {{h^{T}x} + s + v - {{\hat{h}}^{T}x}} \right)\left( {{h^{T}x} + s + v - {{\hat{h}}^{T}x}} \right)^{T}} \right\rbrack}} & (31) \\{{R_{ee}(n)} = {E\left\lbrack {\left( {{h^{T}x} + s + v - {{\hat{h}}^{T}x}} \right)\left( {{x^{T}h} + s^{T} + v^{T} - {x^{T}\hat{h}}} \right)} \right\rbrack}} & (32) \\{{R_{ee}(n)} = {E\left\lbrack \begin{pmatrix}{{h^{T}{xx}^{T}h} + {h^{T}{xs}^{T}} + {h^{T}{xv}^{T}} - {h^{T}{xx}^{T}\hat{h}} +} \\{{{sx}^{T}h} + {ss}^{T} + {sv}^{T} - {{sx}^{T}\hat{h}} +} \\{{vv}^{T} + {vs}^{T} + {{vx}^{T}h} - {{vx}^{T}\hat{h}} -} \\{{{\hat{h}}^{T}{xx}^{T}h} - {{\hat{h}}^{T}{xs}^{T}} - {{\hat{h}}^{T}{xv}^{T}} + {{\hat{h}}^{T}{xx}^{T}\hat{h}}}\end{pmatrix} \right\rbrack}} & (33)\end{matrix}$

Applying independent theory, assuming echo is uncorrelated to the nearend and background noise, the auto correlation of the error signal isgiven by

R _(ee)(n)=h ^(T) R _(xx) h+σ _(s) ₂ +σ_(v) ₂ −h ^(T) R _(xx) ĥ−ĥ ^(T) R_(xx) h+ĥ ^(T) R _(xx) ĥ  (34)

The correlation operations and energy estimates used to form thecorrelation parameters of the present example are measures of coherencebetween the respective signals. For example, a cross correlation betweenthe microphone and error signals is a measure of coherence between themicrophone and error signals. In other examples, other measures ofcoherence could be used that are not mathematical correlations.

Steady State Decision Parameters

The correlation parameters can be used by the controller 202 tocalculate steady state decision parameters for use in the steady statedecision path of the controller. One or more of the following decisionparameters can be calculated at the steady state parameter generator 208of the controller. The following decision parameters exhibit highdynamic range allowing the controller to respond to near-end speech andidentify regions of residual echo in microphone signals that do notcontain near-end speech. This enables the controller to achieve seamlessfull duplex performance under wide-ranging signal conditions.

1. Decision Parameter 1 (DP1)

The decision parameter 1, ξ_(HS) ₁ or DP1 is in the present exampledefined as

$\begin{matrix}{{\xi_{{HS}_{1}}(n)} = \frac{{r_{d\hat{y}}(n)} - {r_{de}(n)}}{{d^{2}(n)}}} & (35) \\{{\xi_{{HS}_{1}}(n)} = \frac{{2\; h^{T}R_{xx}\hat{h}} - {h^{T}R_{xx}h} - \sigma_{s^{2}} - \sigma_{v^{2}}}{{h^{T}R_{xx}h} + \sigma_{s^{2}} + \sigma_{v^{2}}}} & (36)\end{matrix}$

Assuming that during steady state condition, the estimated echo path isnearly equal to the actual echo path. The above equation can besimplified as

$\begin{matrix}{{\xi_{{HS}_{1}}(n)} = \frac{{h^{T}R_{xx}h} - \sigma_{s^{2}} - \sigma_{v^{2}}}{{h^{T}R_{xx}h} + \sigma_{s^{2}} + \sigma_{v^{2}}}} & (37)\end{matrix}$

During far-end single talk σ_(s) ₂ and σ_(v) ₂ becomes zero and theparameter value is close to +1. During near-end speech becomes zero andthe parameter is close to −1. DP1 can be said to be indicative of thepresence of near-end speech.

2. Decision Parameter 2 (DP2)

The decision parameter 2, ξ_(HS) ₂ or DP2 is in the present exampledefined as

$\begin{matrix}\begin{matrix}{{\xi_{{HS}_{2}}(n)} = \frac{{r_{d\hat{y}}(n)} - {r_{de}(n)}}{{r_{d\hat{y}}(n)} + {r_{de}(n)}}} \\{= {\frac{{h^{T}R_{xx}\hat{h}} - \left\lbrack {{h^{T}R_{xx}h} + \sigma_{s^{2}} + \sigma_{v^{2}} - {h^{T}R_{xx}\hat{h}}} \right\rbrack}{{h^{T}R_{xx}\hat{h}} + \left\lbrack {{h^{T}R_{xx}h} + \sigma_{s^{2}} + \sigma_{v^{2}} - {h^{T}R_{xx}\hat{h}}} \right\rbrack}(39)}} \\{= {\frac{{2\; h^{T}R_{xx}\hat{h}} - {h^{T}R_{xx}h} - \sigma_{s^{2}} - \sigma_{v^{2}}}{{h^{T}R_{xx}h} + \sigma_{s^{2}} + \sigma_{v^{2}}}(40)}}\end{matrix} & (38)\end{matrix}$

At steady state, since the echo estimate is equal to the actual echo, wehave

$\begin{matrix}{{\xi_{{HS}_{2}}(n)} = \frac{{h^{T}R_{xx}h} - \sigma_{s^{2}} - \sigma_{v^{2}}}{{h^{T}R_{xx}h} + \sigma_{s^{2}} + \sigma_{v^{2}}}} & (41)\end{matrix}$

During far-end single talk σ_(s) ₂ and σ_(v) ₂ becomes zero and theparameter value is close to +1. During near-end speech R_(xx) becomeszero and the parameter is close to −1. DP2 can be said to be indicativeof the presence of near-end speech.

3. Decision Parameter 3 (DP3)

The decision parameter 3, ξ_(HS) ₃ or DP3 is in the present exampledefined as

$\begin{matrix}{{\xi_{{HS}_{3}}(n)} = \frac{r_{de}(n)}{R_{dd}(n)}} & (42)\end{matrix}$

DP3 is a measure of the proportion of the microphone signal present inthe primary error signal e, and hence of the presence of near-endspeech. From (19) and (28) we have

$\begin{matrix}{{\xi_{{HS}_{3}}(n)} = \frac{{h^{T}R_{xx}h} + \sigma_{s^{2}} + \sigma_{v^{2}} - {h^{T}R_{xx}\hat{h}}}{{h^{T}R_{xx}h} + \sigma_{s^{2}} + \sigma_{v^{2}}}} & (43)\end{matrix}$

At steady state, the echo estimate is equal to the actual echo, so wehave

$\begin{matrix}{{\xi_{{HS}_{3}}(n)} = \frac{\sigma_{s^{2}} + \sigma_{v^{2}}}{{h^{T}R_{xx}h} + \sigma_{s^{2}} + \sigma_{v^{2}}}} & (44)\end{matrix}$

DP3 parameter value during farend single talk is close to 0, duringnear-end the value is +1 and during double talk regions it is close to+1.

4. Decision Parameter 4 (DP4)

The decision parameter 4, ξ_(HS) ₄ or DP4 is in the present exampledefined as

$\begin{matrix}{\xi_{{HS}_{4}} = \frac{R_{ee}(n)}{R_{dd}(n)}} & (45)\end{matrix}$

DP4 is indicative of the size of the error signal, and hence of thepresence of near-end speech. Substituting equations (28) and (34) into(45), we have

$\begin{matrix}{{\xi_{{HS}_{4}}(n)} = \frac{\begin{matrix}{{h^{T}R_{xx}h} + \sigma_{s^{2}} + \sigma_{v^{2}} -} \\{{h^{T}R_{xx}\hat{h}} - {{\hat{h}}^{T}R_{xx}h} + {{\hat{h}}^{T}R_{xx}\hat{h}}}\end{matrix}}{{h^{T}R_{xx}h} + \sigma_{s^{2}} + \sigma_{v^{2}}}} & (46)\end{matrix}$

At steady state, since the echo estimate is equal to the actual echo, wehave

${\xi_{{HS}_{4}}(n)} = \frac{\sigma_{s^{2}} + \sigma_{v^{2}}}{{h^{T}R_{xx}h} + \sigma_{s^{2}} + \sigma_{v^{2}}}$

DP4 parameter value during far-end single talk is close to 0, duringnear-end the value is +1 and during double talk regions it is close to+1.

5. Decision Parameter 5 (DP5)

The decision parameter 5, ξ_(HS) ₅ or DP5 is given by

$\begin{matrix}{{\xi_{{HS}_{5}}(n)} = {\frac{\xi_{{HS}_{4}}(n)}{\xi_{{HS}_{3}}(n)} = \frac{R_{ee}(n)}{r_{de}(n)}}} & (48)\end{matrix}$

Since decision parameter 5 is a ratio between decision parameters 4 and3, the fifth decision parameter may or may not be considered to be adecision parameter in its own right. DP5 parameter value during far-endsingle talk is close to 0, during near-end the value is +1 and duringdouble talk regions it is close to +1.

The use of the steady state decision parameters by the controller isdescribed below.

Transient Decision Parameters

The controller also forms one or more transient decision parameters foruse in the transient decision path of the controller. In the presentexample, the transient decision parameters are calculated throughscaling by the third correlation parameter representing anauto-correlation or energy of the microphone signal. In other examples,the controller could form the transient decision parameters independence on other correlation parameters (or other measures ofcoherence), or without dependence on any of the correlation parameters.

Controller 202 comprises short filters 207 for generating the transientdecision parameters. The short filters are shown in more detail in FIG.4. In the present example a short filter is provided for determiningeach transient decision parameter, but in other examples multipletransient decision parameters could be determined using a single filteror multiple filters could be used in the determination of a singledecision parameter.

1. Transient Decision Parameter 1 (TDP1)

A first transient decision parameter is formed at a first short filterarrangement 401 by a first short adaptive filter 404 arranged togenerate an echo estimate y₁ from a delay compensated far-end signal x′used as its reference input. The far-end signal x is delayed by delayline 403 by a number of samples D, which represents the delay betweenthe source of the echo in the far-end signal and the echo in themicrophone signal. The short filter echo estimate y₁ is subtracted atcanceller 405 from the microphone signal d and the resultant errorsignal e₁ is used to continuously adapt the filter.

The first transient decision parameter in this example is defined as theangle between the vector expressions of the echo estimate y₁ andmicrophone signal d. In regions of the microphone signal that—comprisenear-end speech and/or ambient background noise but no echo of thefar-end signal, this first transient decision parameter is zero. TDP-1can be said to be indicative of the presence of echo in the microphonesignal. Thus, the first transient decision parameter (TDP1) is given by

$\begin{matrix}\begin{matrix}{{\xi_{{HS}\; 1}(n)} = \frac{E\left\lbrack {y_{1} \cdot d^{T}} \right\rbrack}{R_{dd}(n)}} \\{= \frac{E\left\lbrack {\left( {{\hat{h}}_{1}^{T}x^{\prime}} \right)\left( {y + s + v} \right)^{T}} \right\rbrack}{R_{dd}(n)}} \\{= {\frac{E\left\lbrack {\left( {{\hat{h}}_{1}^{T}x^{\prime}} \right)\left( {{h^{T}x} + s + v} \right)^{T}} \right\rbrack}{R_{dd}(n)}(50)}} \\{= {\frac{E\left\lbrack {\left( {{\hat{h}}_{1}^{T}x^{\prime}} \right)\left( {{h^{T}x} + s^{T} + v^{T}} \right)} \right\rbrack}{R_{dd}(n)}(51)}} \\{= {\frac{E\left\lbrack {{{\hat{h}}_{1}^{T}x^{\prime}x^{T}h} + {{\hat{h}}_{1}^{T}x^{\prime}s^{T}} + {{\hat{h}}_{1}^{T}x^{\prime}} + v^{T}} \right\rbrack}{R_{dd}(n)}(52)}}\end{matrix} & (49)\end{matrix}$

Applying independent theory and assuming echo is uncorrelated to thenear end signal and background noise, the equation (52) can be reducedas below

$\begin{matrix}{{\zeta_{{HS}\; 1}(n)} = \frac{{\hat{h}}_{1}^{T}R_{x^{\prime}x}h}{{h^{T}R_{xx}h} + \sigma_{s^{2}} + \sigma_{v^{2}}}} & (53)\end{matrix}$

2. Transient Decision Parameter 2 (TDP2)

A second transient decision parameter is formed at a second short filterarrangement 402 by a second short adaptive filter 406 arranged togenerate an echo estimate y₂ from the echo estimate y generated by theAEEF, which is used as a reference input to the short filter. The shortfilter echo estimate y₂ is subtracted at canceller 407 from themicrophone signal d and the resultant error signal e₂ is used tocontinuously adapt the filter. In this manner, the second filter isarranged to refine the echo estimate generated by the AEEF. The refinedecho estimate is expected to have a high correlation with the microphonesignal during regions of the microphone signal that do not includenear-end speech—e.g. single talk regions in which the microphone signalcomprises background noise and echo alone.

The second transient decision parameter is in this example defined asthe angle between vector expressions of the microphone signal d and therefined echo estimate y₂. In regions of the microphone signalthat—comprise near-end speech and/or ambient background noise but noecho of the far-end signal, this second transient decision parameter iszero. TDP2 can be said to be indicative of the presence of echo in themicrophone signal. Thus, the second transient decision parameter (TDP2)is given by:

$\begin{matrix}\begin{matrix}{{\xi_{{HS}\; 2}(n)} = \frac{E\left\lbrack {y_{2} \cdot d^{T}} \right\rbrack}{R_{dd}(n)}} \\{= \frac{E\left\lbrack {\left( {{\hat{h}}_{2}^{T}y} \right)\left( {y + s + v} \right)^{T}} \right\rbrack}{R_{dd}(n)}} \\{= {\frac{E\left\lbrack {\left( {\hat{h}}_{2}^{T} \right)\left( {{{\hat{h}}^{T}x} + s + v} \right)^{T}} \right\rbrack}{R_{dd}(n)}(55)}} \\{= {\frac{E\left\lbrack {\left( {{\hat{h}}_{2}^{T}\left( {{\hat{h}}^{T}x} \right)} \right) + \left( {{x^{T}h} + s^{T} + v^{T}} \right)} \right\rbrack}{R_{dd}(n)}(56)}} \\{= {\frac{E\left\lbrack {{\hat{h}}_{2}^{T}\left( {{{\hat{h}}^{T}{xx}^{T}h} + {{\hat{h}}^{T}{xs}^{T}} + {{\hat{h}}^{T}{xv}^{T}}} \right)} \right\rbrack}{R_{dd}(n)}(57)}}\end{matrix} & (54)\end{matrix}$

Applying independent theory and assuming echo is uncorrelated to thenear end signal and background noise, the equation (57) can be reducedto

$\begin{matrix}{{\zeta_{{HS}\; 2}(n)} = \frac{{\hat{h}}_{2}^{T}{\hat{h}}^{T}R_{xx}h}{{h^{T}R_{xx}h} + \sigma_{s^{2}} + \sigma_{v^{2}}}} & (58)\end{matrix}$

The first and second short adaptive filters 401 and 402 are shorter thanthe AEEF and operate with a smaller number of filter coefficients thatthe AEEF. Due to the action of the sample rate converter 205, the shortadaptive filters also operate on signals of a lower sampling frequency.It is in general advantageous if secondary adaptive filters used togenerate transient decision parameters are shorter than the primaryadaptive filter which generates the primary echo estimate used to cancelecho in the microphone signal. It is further advantageous if thesecondary filters operate at a lower sampling rate (e.g. due todownsampling of the signals received by the secondary filters, or theselective input of samples into the secondary filters). This allows thecontroller to respond more quickly to changes in the state of themicrophone signal, as a result of quicker convergence with fewercomputations than would be required by a longer filter. This is to betraded-off against filter accuracy which tends to demand a longerfilter. Typically, it can be said that a shorter adaptive filteroperates on each given sample in dependence on fewer previous samples.The first and second short adaptive filters may or may not be of thesame length.

The length of an adaptive filter can be considered to be, for example,the number of samples over which the filter operates, the length of timeover which the filter operates (e.g. the length of time represented bythe number of samples over which the filter concurrently operates), andthe number of coefficients of the filter (typically equal to the numberof samples over which the filter operates). It will be appreciated thatother metrics can be used to define the length of a filter, asappropriate to that particular adaptive filter.

In the example shown in FIG. 2, it has been found that a short filterlength of around 10 ms and an AEEF filter length of around 64 ms workswell when the AEEF operates on signals sampled at 16 kHz and the shortfilters operate on signals sampled at 8 kHz (due to decimation of thesignals received by the short filters by the sample rate converter 205).In this example, the short filters have 80 coefficients and operate over80 samples, and the AEEF has 1024 coefficients and operates over 1024samples. In terms of the lengths of time over which the respectivefilters operate, the length of the short filters is approximately 15% ofthe length of the AEEF. In another example, expressed in terms of thelengths of time over which the respective filters operate, the length ofone or both of the secondary adaptive filters is less than half thelength of the primary adaptive filter. In another example, expressed interms of the lengths of time over which the respective filters operate,the length of one or both of the secondary adaptive filters is less thana quarter the length of the primary adaptive filter. In another example,expressed in terms of the lengths of time over which the respectivefilters operate, the length of one or both of the secondary adaptivefilters is less than a fifth of the length of the primary adaptivefilter. In another example, expressed in terms of the lengths of timeover which the respective filters operate, the length of one or both ofthe secondary adaptive filters is approximately 15% of the length of theprimary adaptive filter.

Use of Decision Parameters at the Controller

The decision parameters generated at the steady state parametergenerator 208 and short filters 207 of the controller are used in thedecision logic 209 of the controller to control activation of theacoustic echo suppressor 203. The decision logic 209 is shown in moredetail in FIG. 3. The decision logic includes convergence detectionlogic 301 which is configured to identify whether the AEEF has convergedand hence identify whether the transient or steady state decision pathsare to be followed. Transient state decision logic 302 is used by thecontroller when the AEEF is not converged. Steady state decision logic303 is invoked when the AEEF is converged.

An overview of the operation of the controller 202 according to thepresent example is shown in FIG. 5 and will now be described. Thecontroller receives audio signals d, e, x and ŷ from an acousticcancellation arrangement comprising an AEEF 201 and to which thecontroller is coupled. If necessary, the audio signals are decimated atsample rate converter 205. The controller then determines by means ofits convergence detection logic whether or not the AEEF is deemed tohave converged. If so, the controller performs control of the AES 203according to its steady state decision path; otherwise, the controllerperforms control of the AES 203 according to its transient statedecision path.

The steady state decision path involves the controller running itssteady state decision logic using steady state decision parametersestimated by the steady state parameter generator 208. The transientstate decision path involves the controller running its transientdecision logic using transient decision parameters estimated by theshort filters 207. In FIG. 5, the estimation of the steady state andtransient parameters is shown as forming part of the steady state andtransient state decision paths, respectively. This is not an indicationof when the decision parameters are in fact estimated. For example,short filters 207 and steady state parameter generator 208 couldcontinuously form and update their respective decision parametersirrespective of the decision path adopted by the controller. Thus, thecontroller need not wait for identification as to whether the AEEF hasconverged in order to perform estimation of the decision parameters.

Once the selected decision path has been performed by the controller andan—output decision is generated, the output of the controller is, ifnecessary, interpolated by the controller so as to convert the samplerate of the controller output to match the sample rate at which the AESoperates. The controller output is then provided to the AES so as tocause the AES to perform echo suppression in accordance with the outputcontrol signal from the controller. In this manner, the controller canactivate and deactivate the AES in dependence on the state of themicrophone signal and the convergence state of the AEEF.

Convergence Detection

Various methods can be used to determine whether the AEEF 201 hasconverged. In the present example, convergence detection logic 301 isused to determine when the AEEF can be said to have converged to arequired level on the basis of a set of counters and predefinedconditions. In other examples, convergence could be judged by a unitexternal to the controller—for example, at the AEEF itself—with thecontroller being arranged to receive a signal indicating the convergencestate of the AEEF and accordingly select one of the transient and steadystate decision paths.

In the present example, convergence detection logic 301 utilizes sixcounters and plurality of predefined conditions for convergencedetection stability. The operation of the convergence detection logic isillustrated by the algorithm shown in FIG. 6. It will be appreciatedthat the convergence detection logic could be configured to operate inaccordance with other algorithms and other counters or predefinedconditions than are used in FIG. 6. The six counters and their use atthe convergence logic will now be described.

The first counter is startup indicator counter, strt_cnt, which is usedas an indicator of initial session timing until the AEEF is converged.In other words, this counter represents a measure of the number ofsamples processed by the controller before convergence of the AEEF canbe considered to have been achieved. To avoid overflow, this counter'smaximum value is typically limited to the length of the register beingused to store the counter.

A second counter is recent noise frame counter noise_cnt, which is ameasure of the number of frames substantially comprising only noisesince the most recent frame comprising near-end speech. The counter isincremented for every noise frame encountered and reset to zero forevery speech frame encountered. For example, if the current frame beingprocessed is a speech frame then this counter will be zero, and if thecurrent frame being processed is the kth noise frame after a group ofone or more speech frames, this counter will be k.

Adaptation counter conv_cnt represents a measure of the number ofsamples in respect of which the controller has activated the AES whenthe AEEF is not expected to have reached a predefined minimumconvergence (i.e. prior to steady and stable convergence of the AEEF).The adaptation counter is used to take decisions at the start ofconvergence of the AEEF.

Suppressor activated counter sp_cnt represents a measure of the numberof samples in respect of which the controller has activated the AES intotal, irrespective of convergence of AEEF.

History counter hist_cnt is a consistency check useful for stable andsteady minimum convergence detection and is used to update conv_cntaccordingly. Hist_cnt is updated if ξ_(HS) ₃ is less than a predefinedthreshold T₃. conv_cnt is updated with hist_cnt only when hist_cntindicates continuous minimum convergence for number of predefinedsamples.

Initial estimation counter init_cnt is a measure of the number ofsamples for which the AEEF has achieved more than the predefined minimumand stable convergence during its adaptation.

The history and initial estimation counters are used for robustestimation of convergence confirmation.

The algorithm performed by the convergence detection logic in thepresent example is illustrated in FIG. 6. This particular algorithmmakes use of the second and third steady state decision parameters, butthis need not be the case and other algorithms are possible fordetermining an indication as to whether the AEEF has converged that donot make use of any decision parameters. The convergence detectionalgorithm shown in FIG. 6 demonstrates excellent performance with thepredefined constants C₁ to C₅ and thresholds T₁ to T₃. Example values ofthese thresholds and constants along with other thresholds used in thetransient and steady state decision logic referred to in FIGS. 6, 7 and8 are shown in Table 1 below.

TABLE 1 Thresholds Value Constants Value T₁ 0.46 C₁ 20 T₂ 0.05 C₂ 1400T₃ 0.2 C₃ 3000 T₄ 0.2 C₄ 64000 T₅ 0.5 C₅ 32000 T₆ −0.5 C₆ 100 T₇ 1.5 C₇540 T₈ 0.2 C₈ 4000 T₉ 0.01 C₉ 540 T₁₀ 0.2

Convergence detection logic 301 is used to identify whether the AEEF hasreached a steady and stable convergence state. Based on this decision,either the steady state decision logic or transient state decision logicis used to generate the decision at the controller as to whether or notto activate the echo suppressor. The output of the convergence detectorlogic is the value conv_flag which identifies whether the AEEF is deemedto be in a converged state: in this example if conv_flag=1 the AEEF isconverged, and if conv_flag=0 the AEEF is not converged. The controllerthen runs either its transient or steady state decision logic independence on whether the AEEF is deemed to have converged to therequired level.

Convergence decision conv_flag is set to 1 when both the countersinit_cnt and conv_cnt are greater than predefined constants C₃ and C₂respectively. If minimum convergence is not achieved within apredetermined number of samples represented by the strt_cnt counter orthe sp_cnt counter (which are independent of AEEF convergence), theconv_flag is forced to 1. This helps to maximise the duplexcharacteristics of the system at which the controller is supported andis especially important for systems in which echo cancellation due tothe AEEF never exceeds the minimum cancellation expected to activateconv_flag. This can be due to various platform issues such as highnon-linearity, frequent flat delay changes, etc. To achieve this,conv_flag is set 1 whenever either of the strt_cnt or sp_cnt are greaterthan predefined constants C₄ and C₅ respectively even though the AEEFhas not in fact reached its steady state.

It will be apparent that the particular thresholds and constants used atthe convergence detection logic depends on the particularcharacteristics of the system at which the controller is supported andof the audio signals on which the controller and AEEF operate. Any otheralgorithm suitable for identifying whether or not the AEEF has convergedcan be used in place of the exemplary algorithm described herein.

Transient State Decision Logic

The transient state decision logic 302 is performed when the AEEF isdeemed not to have converged and operates based on the transientdecision parameters determined by the secondary filters 207. Anexemplary algorithm performed by the transient state decision logic isshown in FIG. 7 and will now be described. It will be apparent that thatthere are many other suitable algorithms for reaching similar decisionswhen the AEEF is in its transient state (i.e. has not converged).

Firstly, the first and second transient decision parameters are comparedto predefined threshold T₄. If either of the transient decisionparameters are greater than predefined threshold T₄, the NLP decision isset to ON (i.e. 1) and the controller activates the AES. If sp_cnt islesser than C₃ samples, the NLP decision is set to ON and the controlleractivates the AES. In this scenario, there may be minor voice clippingbut this is likely to occur only rarely as the AEEF would typicallyrequire only around 500 ms to reach reasonable convergence. This can beuseful to suppress artefacts in the error signal during initialconvergence of the AEEF. If neither of the transient decision parametersare greater than predefined threshold T₄ and sp_cnt is greater than C₃samples, the NLP decision is set to OFF (i.e. 0) and the controller doesnot activate the AES or prevents the AES from activating, as appropriateto the mechanism of control between the controller and AES.

Steady State Decision Logic

The steady state decision logic 303 is performed when the AEEF is deemedto have converged and operates based on the steady state decisionparameters determined by the steady state parameter generator 208. Anexemplary algorithm performed by the steady state decision logic isshown in FIG. 8 and will now be described. It will be apparent that thatthere are many other suitable algorithms for reaching similar decisionswhen the AEEF is in its steady state (i.e. has converged).

The algorithm performed by the steady state decision logic in thepresent example makes use of two counters: a single talk “hangover”counter st_hang_cnt and a double talk “hangover” counter dt_hang_cnt, aswell as a plurality of predefined thresholds for which exemplary valuesare provided in Table 1 above. These counters are used to maintain (i.e.“hangover”) the decision of the controller for a number of samplesindicated by the respective counter following the steady state decisionlogic identifying single talk (ST) or double talk (DT). This helps toavoid frequent fluctuations in the NLP decision of the controller andthereby the associated distortion.

Three types of operations are possible with the hangover counters:

-   -   1) Setting st_hang_cnt or dt_hang_cnt to a predefined value (one        of constants C₆, C₈ and C₉ depending on the point in the        algorithm shown in FIG. 8). When echo alone is identified the ST        hangover is set. When near-end speech alone is identified the DT        hangover is set. This is advantageous because, once echo alone        or near-end speech alone is detected, it is natural for them to        persist for some finite duration.    -   2) Decrementing the counter (provided it has a value greater        than 0) by 1 and forcing the decision to ST or DT based on the        new value of the counter. At no point in time will both counters        have a value greater than 0.    -   3) Breaking the counter by resetting its value to −1. When the        steady state decision logic identifies near-end speech during        ST, st_hang_cnt is reset. When the transient state decision        logic identifies the termination of near-end speech during DT,        dt_hang_cnt is reset. This avoids near-end breaks.

It will be apparent that the particular constants and thresholds usedwill depend on the particular implementation of the steady statedecision logic.

The algorithm comprises three stages that make use of the steady statedecision parameters, including coarse decision making 801, level Idecision confirmation 802 and level II decision confirmation 803. Coarsedecision making makes use of ξ_(HS) ₁ & ξ_(HS) ₂ and also breaks the SThangover counter. Level I decision confirmation makes use of ξ_(HS) ₃and ξ_(HS) ₄ , sets DT hangover and applies ST hangover. Level IIdecision confirmation makes use of ξ_(HS) ₃ , sets ST hangover andapplies DT hangover. Roughly speaking, the coarse decision making stageis sufficient to confirm the presence of far-end speech alone (i.e.single talk, ST). The level I and II decision confirmation stages handledecision making during double talk (DT) when near-end speech is alsopresent, and also perform hangover handling in the manner describedabove. These stages will now be described in more detail.

1) Coarse Decision Making

If either one of ξ_(HS) ₁ and ξ_(HS) ₂ is greater than predefinedthreshold T₅, the NLP decision is set to ON and st_hang_cnt is set to apredefined value C₆. If both ξ_(HS) ₁ and ξ_(HS) ₂ are lesser than orequal to predefined threshold T₅, the NLP decision set to OFF and, ifξ_(HS) ₂ is less than predefined threshold T₆ and startup counterstrt_cnt is greater than predefined value C₄, ST hangover is broken bysetting st_hang_cnt to −1.

2) Level I Decision Confirmation

If ξ_(HS) ₄ is greater than predefined threshold T₇, the NLP decision isreset to ON and DT hangover is broken by setting dt_hang_cnt to −1.Otherwise, (a) ξ_(HS) ₃ is less than predefined threshold T₈ and ST hangover count greater than 0, the NLP decision is reset to ON and ST hangover count is decremented by 1; (b) if ξ_(HS) ₃ is not less thanpredefined threshold T₈, DT hang over count is reset to predefined valueC₇ and ST hang over count is reset to −1.

3) Level II Decision Confirmation

If ξ_(HS) ₃ is less than predefined threshold T₉, the NLP decision isreset to ON, DT hang over count is reset to −1 and ST hang over count isset to pre-defined value C₈. On the failure of the above condition then,based on a comparison of ξ_(HS) ₃ with predefined threshold T₁₀, one ofthe following actions is taken:

-   -   (a) if ξ_(HS) ₃ is greater than predefined threshold T₁₀ and DT        hang over count is greater than 0, DT hang over count is        decremented by 1 and NLP decision is set to OFF;    -   (b) otherwise, the NLP decision is set to ON, DT hang over count        is reset to −1 and ST hang over count is set to predefined value        C₉ if it is lower than that.

The role of the steps in the algorithms shown in FIGS. 7 and 8 can bemore clearly understood by considering the values of the decisionparameters when the microphone signal is in different states. FIG. 9illustrates the variation of microphone and error signals over asequence of samples and the consequent changes in the decisionparameters determined in respect of that signal.

The variation of microphone and error signals in response to a testsignal is shown in FIG. 9( a). Between samples 56500 and 72500 the testsignal comprises near-end speech alone, and between samples 96000 and125000 the test signal comprises both far-end and near-end speech (i.e.double talk). In other regions of the signal, far-end speech only(single talk) is—present.

The corresponding transient decision parameters for the same range ofsamples are shown in FIG. 9( b). Both ξ_(HS) ₁ and ξ_(HS) ₂ have thevalue zero during near-end speech alone and they vary between 0.2 and 1during double talk regions. Their value is near to 1 during single talkregions.

The corresponding steady state decision parameters for the same range ofsamples are shown in FIG. 9( c). The decision parameters ξ_(HS) ₁ andξ_(HS) ₂ have the value close to 1 during single talk regions, they havevalue between 0 and −1 during double talk regions and −1 during near-endspeech alone regions. The decision parameters ξ_(HS) ₃ , ξ_(HS) ₄ andξ_(HS) ₅ have value 1 during near-end speech alone, close to 0 duringsingle talk regions and they have value between 0.2 and 1 during doubletalk regions.

Performance of an Exemplary System

Systems configured in accordance with the teaching herein provide verylow near-end attenuation during double talk regions. For example, thenear-end signal attenuation observed is negligible during normaloperating conditions and is within 6 dB under very low echo to near-end(ENR) signal ratios. Furthermore, the system provides very quickdetection of the onset of near-end and double talk regions. Thisperformance is substantially independent of ENR.

The performance of a system configured in accordance with FIG. 2 andcomprising a controller having logic configured in accordance with theexemplary algorithms described herein is illustrated in FIGS. 10 to 14.The system was provided with test signals comprising speech, CompositeSource Signal (CSS) signals and echo signals generated according to theITU-T G.167/G.168 impulse responses. FIGS. 10 to 14 illustrate ensembleaverages of the test results.

FIG. 10 demonstrates a comparison of the time required for near-endspeech detection between the present system and the conventional methoddescribed in WO 2012/158163 “NON-LINEAR POST-PROCESSING FOR ACOUSTICECHO CANCELLATION” discussed above. It can be seen from the figure thatthe system of FIG. 2 takes about 7 samples to detect near-end speechacross the full range of echo to near-end (ENR) signal ratios, whereasthe conventional method takes about 233 samples for detection of onsetof near-end alone region above around −5 dB ENR, and significantly morethan 233 samples at lower ENR values. The AES controller 202 is veryrobust and doesn't provide any spurious detection under low ENR. Incontrast, the conventional method suffers from increasing near-enddetection delay with decreasing ENR.

FIG. 12 illustrates the time required for the present system to detectnear-end speech at 0 dB ENR. The figure shows—a typical delay of around7 samples from the point at which near-end speech can be observed in thenear-end and error signals.

FIG. 11 provides a comparison of double talk detection delay between thepresent system and the conventional method. It can be observed that thepresent system has a 60 sample delay in detecting the onset of doubletalk whereas the conventional method suffers from a 560 sample delay at0 dB ENR, which increases with decreasing ENR. FIG. 13 illustrates thetime required for the present system to detect double talk at 0 dB ENR.The figures show the typical delay of around 60 samples from the pointat which double talk can be observed in the near-end and error signals.

FIG. 14 shows the near-end signal attenuation during double talk of thepresent system and a system configured in accordance with theconventional method. It can be observed that the present system hasminimum attenuation of 0.1 dB even under low ENR cases. In contrast, theconventional method attenuates near-end signal heavily under low ENRcases and achieves a minimum attenuation of only 6 dB at 10 dB ENR.Higher attenuation of the near-end signal during double talk regionsleads to voice breaks or clipping. This figure therefore illustrates theexcellent full duplex performance of the present system in comparison tothe conventional method across the full range of ENR, and especiallyunder low ENR conditions.

The controller of FIG. 2 and its aspects as in FIGS. 3 and 4 are shownas comprising a number of functional blocks. This is for illustrativepurposes only and is not intended to define a strict division betweendifferent parts of hardware on a chip or between different programs,procedures or functions in software. The term logic as used herein canrefer to any kind of software, hardware, or combination of hardware andsoftware.

Controllers configured in accordance with the present invention could beembodied in hardware, software or any suitable combination of hardwareand software. A controller of the present invention could comprise, forexample, software for execution at one or more processors (such as at aCPU and/or GPU), and/or one or more dedicated processors (such asASICs), and/or one or more programmable processors (such as FPGAs)suitably programmed so as to provide functionalities of the controller,and/or heterogeneous processors comprising one or more dedicated,programmable and general purpose processing functionalities. Inpreferred embodiments of the present invention, the controller comprisesone or more processors and one or more memories having program codestored thereon, the data processors and the memories being such as to,in combination, provide the claimed controller and/or perform theclaimed methods.

The term software as used herein includes executable code for processors(e.g. CPUs and/or GPUs), firmware, bytecode, programming language codesuch as C or OpenCL, and modules for reconfigurable logic devices suchas FPGAs. Machine-readable code includes software and code for defininghardware, such as register transfer level (RTL) code as might begenerated in Verilog or VHDL.

Any one or more of the algorithms and methods described herein could beperformed by one or more physical processing units executing programcode that causes the unit(s) to perform the algorithms/methods. The oreach physical processing unit could be any suitable processor, such as aCPU or GPU (or a core thereof), or fixed function or programmablehardware. The program code could be stored in non-transitory form at amachine readable medium such as an integrated circuit memory, or opticalor magnetic storage. A machine readable medium might comprise severalmemories, such as on-chip memories, computer working memories, andnon-volatile storage devices.

The applicant hereby discloses in isolation each individual featuredescribed herein and any combination of two or more such features, tothe extent that such features or combinations are capable of beingcarried out based on the present specification as a whole in the lightof the common general knowledge of a person skilled in the art,irrespective of whether such features or combinations of features solveany problems disclosed herein, and without limitation to the scope ofthe claims. The applicant indicates that aspects of the presentinvention may consist of any such individual feature or combination offeatures. In view of the foregoing description it will be evident to aperson skilled in the art that various modifications may be made withinthe scope of the invention.

1. A controller for an echo suppressor configured to suppress a residualecho of a far-end signal included in a primary error signal, thecontroller adapted for operation with a primary adaptive filterconfigured to form a primary echo estimate of the far-end signalincluded in a microphone signal and with an echo canceller configured tocancel that primary echo estimate from the microphone signal so as toform the primary error signal, the controller comprising: a secondaryadaptive filter configured to form a secondary echo estimate of thefar-end signal included in the microphone signal; a coherence estimatorconfigured to form a first measure of coherence between the microphonesignal and the primary error signal, and a second measure of coherencebetween the microphone signal and the primary echo estimate; and controllogic being configured to: in a first mode selected when the primaryadaptive filter is in a non-converged state, combine the microphonesignal and the secondary echo estimate so as to form a transientdecision parameter indicative of a state of the microphone signal and,in dependence on the transient decision parameter, control activation ofthe echo suppressor; and in a second mode selected when the primaryadaptive filter is in a converged state, combine the first and secondmeasures of coherence so as to form one or more first steady statedecision parameters indicative of a state of the microphone signal and,in dependence on said one or more first steady state decisionparameters, control activation of the echo suppressor.
 2. A controlleras claimed in claim 1, the echo suppressor being configured to, whenactivated, replace the primary error signal with generated noise havingcharacteristics selected to substantially match background noisecomprised in the microphone signal.
 3. A controller as claimed in claim1, wherein the transient decision parameter is indicative of thepresence of echo of the far-end signal in the microphone signal.
 4. Acontroller as claimed in claim 1, the control logic being furtherconfigured to, in its first mode, cause activation of the echosuppressor when the transient decision parameter indicates that echo ofthe far-end signal is present in the microphone signal but near-endspeech is not present.
 5. A controller as claimed in claim 1, thecontrol logic being further configured to, in its first mode, combinethe microphone signal and the secondary echo estimate such that thetransient decision parameter represents a measure of angle between avector expression of the microphone signal and a vector expression ofthe secondary echo estimate, said vector expressions each being asequence of samples of the respective microphone or secondary echoestimate.
 6. A controller as claimed in claim 1, the secondary adaptivefilter being further configured to form the secondary echo estimate fromthe far-end signal or the primary echo estimate and being adapted independence on a second error signal generated by means of a comparisonof its secondary echo estimate and the microphone signal.
 7. Acontroller as claimed in claim 1, the coherence estimator being furtherconfigured to form a third measure of coherence of the microphone signalwith itself, and the control logic being configured to, in its firstmode, combine the microphone signal and the secondary echo estimate independence on the third measure of coherence.
 8. A controller as claimedin claim 7, wherein the third measure of coherence is a measure ofautocorrelation or energy of the microphone signal.
 9. A controller asclaimed in claim 1, the control logic being further configured to, inits second mode, cause activation of the echo suppressor when the one ormore first steady state decision parameters indicate that near-endspeech is not present.
 10. A controller as claimed in claim 1, whereinthe first measure of coherence is a measure of cross correlation betweenthe microphone signal and the primary error signal, and the secondmeasure of coherence is a measure of cross correlation between themicrophone signal and the primary echo estimate.
 11. A controller asclaimed in claim 1, the control logic being further configured to, inits second mode, combine the first and second measures of coherence soas to form a first one of the one or more first steady state decisionparameters proportional to a difference between the first and secondmeasures of coherence, said difference being scaled by a measure of themagnitude of the microphone signal.
 12. A controller as claimed in claim1, the control logic being further configured to, in its second mode,combine the first and second measures of coherence so as to form asecond one of the one or more first steady state decision parametersproportional to a difference between the first and second measures ofcoherence, said difference being scaled by a sum of the first and secondmeasures of coherence.
 13. A controller as claimed in claim 1, thecoherence estimator being further configured to form a third measure ofcoherence of the microphone signal with itself, and the control logicbeing configured to, in its second mode, combine the first and thirdmeasures of coherence so as to form a second steady state decisionparameter indicative of a state of the microphone signal and to controlactivation of the echo suppressor further in dependence on the secondsteady state decision parameter.
 14. A controller as claimed in claim13, the coherence estimator being further configured to form a fourthmeasure of coherence of the primary error signal with itself, and thecontrol logic being configured to, in its second mode, combine the thirdand fourth measures of coherence so as to form a third steady statedecision parameter indicative of a state of the microphone signal and tocontrol activation of the echo suppressor further in dependence on thethird steady state decision parameter.
 15. A controller as claimed inclaim 1, the control logic being further configured to, in its secondmode, cause activation of the echo suppressor when the second and/orthird steady state decision parameters indicate that near-end speech isnot present.
 16. A controller as claimed in claim 1, the secondaryadaptive filter being further configured to operate at a lower samplingrate than the primary adaptive filter.
 17. A method for controlling anecho suppressor configured to suppress a residual echo of a far-endsignal included in a primary error signal received from an echocanceller, the echo canceller being configured to cancel a primary echoestimate from a microphone signal so as to form the primary errorsignal, the primary echo estimate being formed at a primary adaptivefilter and representing an estimate of the far-end signal comprised inthe microphone signal, the method comprising: at a secondary adaptivefilter, forming a secondary echo estimate of the far-end signalcomprised in the microphone signal; forming a first measure of coherencebetween the microphone signal and the primary error signal, and a secondmeasure of coherence between the microphone signal and the primary echoestimate; determining a convergence state of the primary adaptivefilter; and selecting in dependence on the determined convergence state:a transient decision path if the primary adaptive filter is determinedto be in a non-converged state, the transient decision path combiningthe microphone signal and the secondary echo estimate so as to form atransient decision parameter indicative of a state of the microphonesignal; a steady state decision path if the primary adaptive filter isdetermined to be in a converged state, the steady state decision pathcombining the first and second measures of coherence so as to form oneor more first steady state decision parameters indicative of a state ofthe microphone signal; and controlling activation of the echo suppressorin dependence on said transient or one or more first steady statedecision parameters.
 18. A method as claimed in claim 17, wherein thestep of forming a secondary echo estimate comprises: at the secondaryadaptive filter, forming the secondary echo estimate from the far-endsignal or the primary echo estimate; and adapting the secondary adaptivefilter in dependence on a second error signal generated by means of acomparison of its secondary echo estimate and the microphone signal. 19.A non-transitory computer readable storage medium having stored thereoncomputer readable instructions that, when executed at a computer systemfor generating a representation of a digital circuit from definitions ofcircuit elements and data defining rules for combining those circuitelements, cause the computer system to generate, for operation with aprimary adaptive filter configured to form a primary echo estimate of afar-end signal included in a microphone signal and with an echocanceller configured to cancel that primary echo estimate from themicrophone signal so as to form a primary error signal: a secondaryadaptive filter configured to form a secondary echo estimate of thefar-end signal included in the microphone signal; a coherence estimatorconfigured to form a first measure of coherence between the microphonesignal and the primary error signal, and a second measure of coherencebetween the microphone signal and the primary echo estimate; and controllogic being configured to: in a first mode selected when the primaryadaptive filter is in a non-converged state, combine the microphonesignal and the secondary echo estimate so as to form a transientdecision parameter indicative of a state of the microphone signal and,in dependence on the transient decision parameter, control activation ofthe echo suppressor; and in a second mode selected when the primaryadaptive filter is in a converged state, combine the first and secondmeasures of coherence so as to form one or more first steady statedecision parameters indicative of a state of the microphone signal and,in dependence on said one or more first steady state decisionparameters, control activation of the echo suppressor
 20. Anon-transitory computer readable storage medium having stored thereoncomputer executable instructions that when executed cause at least oneprocessor to control an echo suppressor configured to suppress aresidual echo of a far-end signal included in a primary error signalreceived from an echo canceller, the echo canceller being configured tocancel a primary echo estimate from a microphone signal so as to formthe primary error signal, the primary echo estimate being formed at aprimary adaptive filter and representing an estimate of the far-endsignal comprised in the microphone signal, by: at a secondary adaptivefilter, forming a secondary echo estimate of the far-end signalcomprised in the microphone signal; forming a first measure of coherencebetween the microphone signal and the primary error signal, and a secondmeasure of coherence between the microphone signal and the primary echoestimate; determining a convergence state of the primary adaptivefilter; and selecting in dependence on the determined convergence state:a transient decision path if the primary adaptive filter is determinedto be in a non-converged state, the transient decision path combiningthe microphone signal and the secondary echo estimate so as to form atransient decision parameter indicative of a state of the microphonesignal; a steady state decision path if the primary adaptive filter isdetermined to be in a converged state, the steady state decision pathcombining the first and second measures of coherence so as to form oneor more first steady state decision parameters indicative of a state ofthe microphone signal; and controlling activation of the echo suppressorin dependence on said transient or one or more first steady statedecision parameters.